Epstein-Barr virus has been demonstrated clinically to precede some cases of aplastic anemia. Documentation of EBV infection was previously based on the clinical syndrome and viral serologies. We have undertaken a systematic search for EBV within the marrow of patients with aplastic anemia. Using a combination of immunofluorescence for viral antigens, in situ hybridization we have identified four patients with large quantities of EBV in their marrow but not peripheral blood. Using an in vitro hematopoietic colony assay, we have demonstrated that EBV is found within the erythroid colonies and is toxic to these colonies. Further studies are currently underway to elucidate the mechanism of toxicity to bone marrow.